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A large portion of the central and northeastern U.S. is under the threat of severe weather today, as the powerful storm system responsible for this multi-day outbreak continues to migrate east. Areas of central Texas and western Oklahoma all the way to the Hudson River Valley in New York are under the threat of severe weather, with the key area of interest today being across portions of northeast Texas into the Mid-South, where an outbreak of severe storms capable of strong, long-tracked tornadoes, extremely large hail and destructive winds is expected later this afternoon into the evening. The threat will intensify and shift east into the Tennessee and Ohio Valley tomorrow. Very strong directional wind shear and strong instability are expected to be in place across the Arklatex later this afternoon, providing an environment very favorable for intense supercell thunderstorm development. Unfortunately, this next wave of thunderstorm activity will likely be developing over areas that were ravaged yesterday, namely southern and central Arkansas, where several highly destructive tornadoes occurred last evening. (Click here for the latest details on the April 25, 2011 Arkansas tornadoes.) This is a very serious weather situation, and as always, residents in the risk areas are strongly encouraged to closely monitor local media for watches and warnings. Despite the less than ideal terrain, a number of chasers will be streaming this event live as well, providing much needed help with ground-truth for local television and the NWS.

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A very special set of meteorological parameters are coming together over the Mid-South, setting the stage for what is expected to be a very rough afternoon across Mississippi, Alabama, Tennessee and surrounding states to the north and east; the threat for tornadic storms will extend literally from the Gulf of Mexico all the way into northern New York and Canada by late evening. Extreme wind shear--the result of intense winds intersecting at various levels of the atmosphere, which today is especially pronounced in the lower levels--and moderate to high instability will be in place over the same areas, providing what can only be described as an ideal environment for the development of thunderstorms capable of producing tornadoes. This combination of shear and instability can be presented as the Energy Helicity Index, or EHI, also at left. (Click here for more on the significance of EHI.) As such, the Storm Prediction Center has issued another HIGH RISK for severe weather across this area, including a rare 45% hatched area for tornadoes (see graphic at left). Of particular concern is the potential for long-tracked, strong to violent tornadoes (hence the current PDS Tornado Watches found here). There will be a number of chasers covering this extremely dangerous weather event today, providing live video streams for local media and NWS to monitor so they can see conditions on the ground in real time. As always, those in the risk areas are urged to keep themselves safe by being prepared and aware of this serious weather situation, and have a plan in place in case of warnings.

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Wednesday, April 27, 2011, was that once in a generation tornado outbreak that's been looming since Super Outbreak April 1974. With the death toll climbing over 200 as of this morning, it is easily the deadliest outbreak in the modern Doppler era. Seemingly every thunderstorm that formed after the noon hour spawned a violent tornado. Dozens of communities in Mississippi, Alabama and Georgia took direct hits. The damage is staggering. Seasoned, veteran storm chasers and meteorologists agree that they have never seen anything like this.

Above are high resolution radar images from Tuscaloosa, Alabama, captured as the almost otherworldly tornado was tearing through the city. Reed, Joel and Chris observed this tornado just after its birth, but were unable to keep up with the storm due to an unfavorable road network and the high speed at which the storm was traveling. Based on the videos from various angles, I am willing to call the most incredible tornado I have ever seen.

Something very fascinating which stood out about yesterday's event is that, aside from their very violent nature, so many of the tornadoes had numbers of horizontal vortices coming from the sides of the tornadoes themselves, as well as satellites from above. This is not something commonly seen, with maybe one or two tornadoes per year displaying these characteristics, and usually more subtly than the examples from yesterday. Obviously, the environment over northern and central Mississippi, Alabama and Georgia was perfect for these kinds of freak tornadoes. At one point yesterday afternoon, Storm Relative Helicity for the north-central Alabama area was coming in at &gt;1,000 m2/s2. That index in itself is rather extravagant, but when you add in CAPE at &gt; 3000+ j/Kg, the results can be horrifying, as we are now seeing. Below is Reed, Chris and Joel's video.

Beyond all of the destruction and loss of life, tornado outbreaks like this are also invaluable learning tools. People naturally want to dissect any given event to determine what worked, what could have been done differently, and to use what they learn to devise ways of doing better in the future. As it applies to yesterday, it is easy to say that National Weather Service personnel, members of the media, and storm chasers and spotters all worked in unison to provide and disseminate information that resulted in the best-possible warnings to those in harm's way, given the tools at their disposal. Thousands of people would have likely succumbed to the wrath of yesterday's storms without their efforts. Developing better warning systems is always something people like to talk about, but the real challenge, obviously, is how? It seems that we may have reached the limit of what's possible with Doppler radar and human-based warnings. One new method that is being explored right now involves infrasound technology. Tupelo, MS-based Hyperion Technology Group (who you may recognize as the team who builds the radar and cannon array for the Dominator) was in the field yesterday, in partnership with TVN, in an effort to collect some initial data in support of a tornado infrasound research mission. One of the primary goals of this research is to find and sample a detectable infrasound signal produced by a tornado and received at various ranges away from the tornado (i.e., essentially a warnable signal that a tornado has formed or is imminent). They intercepted two tornadoes, including the Houston/Okolona, MS tornado, which was large and strong. (This same storm eventually produced the Smithville, MS tornado, which, evidently, was violent based on Hyperion's assessment of damage afterward.) They successfully deployed sensors on a subsequent supercell, around the time of the Tuscaloosa tornado (but not in the immediate vicinity of that storm). So, in the middle of yesterday's catastrophic outbreak, some very cutting-edge science was being was being put to use, with hopes that something can be gained to allow the meteorology and engineering community to build on finding that "something better" for devising warnings. We'd consider Hyperion's efforts a very successful trial run for this new research project, and are looking forward to being able to perform multiple data collects as the heart of the tornado season is upon us. Below are some radar images from the initial storm Hyperion was sampling.

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Damage surveys continue across the south today as engineers and meteorologists labor to assess the magnitude of the destruction left in the wake of Wednesday's tornadoes. So far, at least one of those tornadoes has been assigned the highest ranking on the Enhanced Fujita scale. Officials determined early yesterday thatEF-5 damage occurred in the town of Smithville, Mississippi (pop. ~ 800). This storm also claimed 14 lives and injured at least 40. This is the same storm noted in the last blog post, which the team from Hyperion Tech Group was targeting for their infrasound project. See their video, below.

Elsewhere, an EF-4 rating has been assigned to the very damaging Catoosa County/Ringgold, GA tornado, while two separate EF-4 designations have been made on tornadoes that occurred in Tennessee. Assessments are still ongoing across Alabama (a process that will likely take several days or even weeks to complete) and other states; for the latest on damage assessments across the southern and eastern U.S., visit the National Weather Service website. We will continue to update as new information becomes available on this unbelievable event.